Composite materials are used in ever increasing quantities in a wide variety of industries. For example, commercial aircraft incorporate composite materials in primary and secondary structure due to the favorable mechanical properties of composite materials. Such favorable properties may translate into reduced weight of the aircraft and increased payload capacity and fuel efficiency. Furthermore, composite materials may provide an extended service life for the aircraft as compared to aircraft formed of conventional metallic construction.
Rework of composite structures is occasionally required in order to remove or compensate for inconsistencies in the composite structure. Such inconsistencies may comprise localized areas that may not conform to product and/or performance specifications for any one of a variety of reasons. For example, areas of the structure may be out-of-tolerance with product specifications because of inconsistencies such as, without limitation, voids, dents, delaminations and porosity in the composite structure.
The reworking of the composite structure may include removing a portion of the composite structure containing the inconsistency and replacing the removed material with a patch. Alternatively, reworking of the composite structure may include bonding a patch to the outer mold line of the structure over the area containing the inconsistency. The patch may be formed as a stack of plies of composite material which may be of the same or different type of material from which the composite structure is formed.
Reworked structures in commercial aircraft must typically meet federally-mandated strength requirements. Such strength requirements may include demonstrating that a reworked structure in an aircraft possesses the same strength characteristics that the composite structure possesses in its original condition. For metallic structures, a reworked area is typically visually inspectable such as from an exterior side of the structure to confirm the quality of the reworked structure. For example, a reworked metallic skin may include the installation of a splice that may be mechanically fastened to the skin with fasteners such as rivets which are visually inspectable to verify the integrity of the rivets and the splice.
However, the reworking of composite structures typically includes the use of adhesives such as epoxy for bonding a patch to a composite structure at a rework area. Because the entire adhesive bond joint that bonds the patch to the structure is typically not visually observable, it may not be possible to fully visually verify the integrity of the bonded joint between the patch and the structure. Furthermore, the durability of the bonded joint between the patch and the composite structure may not be readily determinable. Even further, conventional process controls for reworking of composite structures may limit the ability to confirm the integrity of the reworked composite structure.
In light of the above-noted limitations associated with inspection of bonded joints and in order to meet federally-mandated requirements for physically reworking of composite structures, conventional reworking practices may avoid complete reliance on the bonded joint as the primary load path and may rely on secondary load paths for certifying the bonded joint. In addition, the bonded joint between a patch and the composite structure may rely on mechanical fasteners installed around a perimeter of the patch. The mechanical fasteners may act as an arrestment mechanism to prevent peeling forces at the patch perimeter from exceeding the strength capabilities of the adhesive.
Unfortunately, the conventional practice of reworking primary structure may require specific authorization to perform the rework and may require supervision of the rework by an engineering authority. In addition, the reworking of primary structure may require highly-skilled technicians to perform the rework in a controlled environment. Furthermore, conventional practices for reworking composite structures such as by installing mechanical fasteners in the patch as crack arrestors may defeat the original purpose and intent of fastener-free composite structural design. Even further, the reliance on secondary load paths and the installation of mechanical fasteners in the bonded joint may increase the cost and complexity of the rework and result in an increase in weight.
As can be seen, there exists a need in the art for a system and method for reworking of a composite structure using a bonded patch that meets predetermined strength requirements. Furthermore, there exists a need in the art for a system and method for reworking a composite structure that provides improved confidence in the integrity of the bonded patch without the need for fasteners or reliance on secondary load paths. In this regard, there exists a need in the art for a design approach for reworking of composite structures that results in a predictable and fail-safe operating life of the reworked structure.